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If the cheating was a one-time thing in the past, and she's having no contact with the person she cheated with, she should probably not tell him about it. It will only serve to justify her husband's cheating, and increase the drama of the situation.
But she should make sure that from now on her behavior is the same she expects from her husband.
Your friend should get some help, because with cheating on both sides, there are major problems in this relationship.